Oxide gabbros are a minor but diffuse component of the lower oceanic crust. Their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because ...they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models. Here, we report on a study of lower-crust gabbros recovered from the Vema Lithospheric Section at 11°N along the Mid Atlantic Ridge, where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusives and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in Fe-Ti-oxides and apatites. Their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of Fe-Ti-P saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing, de-compaction, mineral resorption and chemical re-equilibration. Melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment. We propose a regime of lateral, instead of vertical, melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of Fe-Ti-P melt pockets in oceanic domains.
•Anomalously abundant apatite-rich ferrodiorites and evolved gabbros are sampled at the Vema Lithospheric Section (MAR).•Ferrodiorites are hybrid rocks formed by injection of nelsonitic melts at the ductile/brittle transition.•Strong melt differentiation results from lateral melt percolation possibly fostering fluid immiscibility.•Melt injection is controlled by the ridge parallel normal faults rooting close to the axial magma chamber.•Major fault slip events generate melt squeezing, overpressurized injection, cataclasis and resorption during relaxation.
The presence of zircon xenocrysts in Mid Atlantic Ridge (MAR) rocks poses essential questions concerning the path they have followed from their sources, how they have survived transport through the ...mantle, and why they are much more abundant than in any other ocean. Sixteen out of twenty-five seafloor samples collected between S 21° to N 42° contained xenocrysts, here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the Atlantic seafloor age from which they were collected. The xenocrysts are of two types, young and old. Hf and O isotopes and trace elements indicate that the younger crystals (3.1 Ma to 25.6 Ma) are oceanic, derived from previous MAR rocks, whereas the older crystals (190 Ma to 3.2 Ga) are continental, derived from crust slivers located at the MAR or immediate vicinity. The high ∂18O (~7‰ to 11‰) of many older xenocrysts indicates ultrafast recycling through the mantle; otherwise, the fast diffusivity of oxygen would have driven the xenocrysts ∂18O to that of their mantle hosts, ∂18O ≈ 5.3‰. The incorporation of xenocrysts into MAR magmas mainly occurs after ridge jumps, when hot mafic magma pervades a new crustal section scavenging xenoliths, or locally melts felsic wall-rocks and mixes with the resulting zircon-laden low-T magmas. In either case, the fusible component of the xenocrysts source contaminates the upwelling mafic magma. When the source was oceanic, the resulting chemical and isotopic signatures of the contaminated magma would remain MORB-like, albeit enriched in incompatible elements. When the source was continental, the resulting magma would reflect a perceptible continental influence. Crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the MAR magmas compared with other MORBs. These features are ultimately related to the Atlantic slow-spreading because it promotes ridge jumping and, hence, active crustal recycling.
•Zircon xenocrysts abound in the Mid Atlantic Ridge, more than in other oceanic ridges.•This indicates active crust recycling caused by the Atlantic slow-spreading.•Xenocrysts younger than 26 Ma derived from previous MAR rocks, likely plagiogranites.•Xenocrysts older than 190 Ma, derived from continental slivers preserved at the MAR.•The incorporation of xenocrysts impacts notably the chemistry of the MAR magmas.
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